An oil pump, in other words a hydraulic pump, which supplies an operating fluid to an automatic transmission, is driven by an engine. Therefore, while a lock-up clutch is being disengaged, or during a coast down state, an engine rotational speed (i.e., a rotational speed of a crankshaft) falls down about to a level of an idling rotational speed, by which the oil pump is disable to discharge an operating fluid at an adequate amount, and further it may become difficult to assure an enough level of an operating fluid pressure. JP1986(61)-23485U discloses an oil pump apparatus provided with two integral gear pumps. This oil pump apparatus assures an amount of discharged oil at an engine low rotational speed range, and generates a desired level of operating fluid pressure. According to this oil pump apparatus, at an engine high rotational speed range, only one of the two gear pumps are driven so that it is possible to avoid an occurrence of an excessive oil pressure. Therefore, superior work efficiency can be expected.
As described above, an amount of discharged oil depends on an engine rotational speed. When an activation of an engine is discontinued, even by the oil pump apparatus disclosed above, it may be difficult to assure an oil pressure at a level which is required to control a shift operation or a lock-up clutch. As is obvious from that, an oil pressure level varies depending on whether any operations, by which an engine rotational speed drops suddenly, are implemented in response to a shift operation, or otherwise, an engine rotational speed-dependency of an amount of discharged oil contributes to complicating a correction of control parameters which are referred to at a shift operation control or a lock-up clutch control.
In the light of the foregoing, another type of oil pump apparatus has been known, in which electrically driven oil pumps, which are arranged in parallel, are capable of being operated as an auxiliary oil pump, for the purpose of compensating for an amount of discharged oil at a time that an engine rotational speed drops, or an engine is stopped. According to this oil pump apparatus, an entire structure may become complicated, and additional equipments, such as a motor pump, may be required, which may not be able to solve the above matters. Moreover, if a vehicle is being driven, it is more economical to consume a vehicle driving energy, in terms of generating an oil pressure. Still moreover, a positive vehicle driving method may be proposed, a method by which a vehicle inertia driving is performed more frequently for the purpose of restraining a fuel consumption rate.
The present invention has been made in view of the above circumstances, and provides an automatic transmission apparatus by which an oil pump is capable of being operated by a driving power source that is different from an engine being rotated.